The present invention relates to a method for forming MOS transistors, and to transistors formed by this method.
A difficulty in the fabrication of small-geometry MOS devices is the following quandary: If the source/drains are doped to a level high enough to give reasonably low series resistance, the magnitude of the electric field in the channel adjacent to the drain during the on-state will be tremendous, and hot carriers and impact ionization effects will become a nuisance.
As the MOSFET channel becomes shorter, the electric field along the channel becomes more important. That is, the potential distribution becomes two dimensional, and the effect of the electric field along the channel can no longer be ignored while considering the effects of the electric field normal t o the channel. In particular, where the electric field along the channel is high, as it is likely to be in short-channel MOS devices, impact ionization is likely to occur near the drain. The resulting substrate current greatly increases the likelihood of latch-up. In addition, hot carriers are also likely to be generated by the strong electric field magnitude near the drain, and some of these carriers may be injected into the oxide, leading to threshold shift.
Thus, it is an object of the present invention to reduce the magnitude of electric field seen along the channel near the drain of an MOS device.
A difficulty in fabricating MOS devices with reasonably shallow source/drain diffusions is that the sharp curvature of the junction will cause a locally increased electric field, and therefore a reduced breakdown voltage. This causes great difficulty when high-voltage devices with reasonably small geometry are desired. If an arsenic implanted drain junction is to have a reasonable curvature, a very long drive-in time is required, causing undesired thermal effects in the other parts of the device.
Thus, it is a further object of the present invention to provide a method for forming source/drain regions having low series resistance and a large junction radius.
It is a further object of the present invention to provide a method for forming source/drain regions having a low contact resistance and a large junction radius.
It is a further object of the present invention to provide a process, satisfying the above-stated objects, which does not require additional masking or heating steps.